lol, I just went through this. It all depends, old isn't bad. In many ways it is good, a mature has already gone through it's nitrogen cycle. It may have a small cycle when moved, but there are other factors as I have recently learned.
Care- Has the tank been maintained properly. I would advise taking a test kit and checking the water, see that everything is in order.
Condition- Does all of the equipment work, check EVERYTHING. Be cautious, if they say a bulb is burned, make them connect one that does work to show you the ballast indeed works. Go over the pump, sump, skimmer, look for anything wrong. Verify that everything works. See that the stand and canopy are in good condition and don't have wood rot. Check the seals for any sign of leaking.
If everything checks out, then age shouldn't matter. In fact it can be a good thing. If you decide to buy the tank then be prepared. Here is how I would break down the set up.
Take plenty of buckets with lids. When you arrive, disconnect all power and equipment. Remove the canopy (If applicable), any lights, and get all electronics out of the way. Begin draining the water, take out about 30-50% of the water and stop. Now remove the rock and ensure it stays submerged in saltwater, seal the buckets with lids. I marked which buckets had rock and which didn't. Capture any livestock and put them in a container, if you use a bucket, have an air stone and heater ready to use. Finish draining the tank. I would throw out the substrate and get fresh sand, or whatever you choose.
Set the tank up as you would if it was new. Check the levels carefully in the event you do experience a cycle.